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Or printed directly to the PCB, if it's so very important.
That would make sense wouldn't it? Stickers falling off isn't uncommon which is a bit daft really and shouldn't give manufacturers a 'get out of jail free' card. If they don't take adequate steps to keep them in place then the onus should be on them to prove the card is out of warranty.
Question for the OP - what card do you have? Presumably companies like XFX cover their cards for 1-2 years so if it's something fairly recent (e.g. 7850) that's only been on the market for < 1 year then the lack of a sticker is kind of academic...regardless of when it was bought it'd still be within the warranty. Whilst XFX might still not play ball it'd hopefully give you the option to pursue them via other channels e.g. small claims.
Might be worth ringing the Citizens Advice consumer helpline (they took over from Consumer Direct earlier this year) to see what they can suggest and to clarify your rights.